(1) Field of the Invention
The present invention relates to a method of manufacturing .beta. type silicon nitride whiskers, and particularly to a method of manufacturing .beta. type silicon nitride whiskers by which well developed .beta. type silicon nitride whiskers having a long fiber length can be easily produced at a relatively low temperature of 1,250.degree.-1,450.degree. C.
(2) Description of the Prior Art
In general, the application of .beta. type silicon nitride whiskers has been eagerly expected as a fiber reinforcement for a composite material because of excellent high temperature strength and thermal shock resistance of silicon nitride.
As the prior techniques relating to the present invention, there are the following methods, but they are quite different from the present invention for the reasons also stated below. Therefore, the invention cannot easily be inferred therefrom. The invention is to provide a method of commercially advantageously producing .beta. type silicon nitride whiskers.
(A) Japanese Patent Application Publication No. 12,320/1976 "A Method of Manufacturing Silicon Nitride"
This reference is similar to the present invention in view of addition of a fluoride to a mixture of silica and carbon and in the composition thereof, but is different from the present invention in a way of adding the fluoride. Further, this reference is completely silent about the molar ratio of silica, carbon and cryolite, and it also fails to refer to the crystal type of the whiskers.
(B) Japanese Patent Application Publication No. 21,160/1975 "A Method of Manufacturing Fibrous Silicon Nitride Crystals"
This is quite different from the present invention in that silicon is used as the source of Si of silicon nitride.
(C) Japanese Patent Application Publication No. 4,480/1975 "A Method of Manufacturing Silicon Nitride Fibers"
This reference uses a metal additive as a catalyst of the nitriding reaction, and therefore it quite differs from the present invention. This reference is utterly silent about the crystal form of the obtained whiskers.
(D) Japanese Patent Application Publication No. 27,755/1974 "A Method of Manufacturing Silicon Nitride Whiskers"
This reference adds Cl component in N.sub.2 atmosphere, and therefore it differs from the present invention. Further, it does not to the crystal form of the produced whiskers at all.
Briefly speaking, there have been conventionally known the following methods as the method of manufacturing the silicon nitride whisker.
(a) Direct nitriding of metallic silicon powder.
(b) Catalytic reaction of silicon halide with ammonia.
(c) In a silica reduction method, a fluoride or chlorine component is added, and silica is reacted with N.sub.2 gas.
According to the methods (a) and (b), merely powdery whiskers having a short fiber length can be obtained. According to the method of (c), although the whiskers of a slightly longer fiber length can be obtained, the structure is mainly of .alpha. type when the treating temperature is a relatively low temperature of 1,300.degree. to 1,400.degree. C.
In the conventional techniques, a problem is the phase transition of Si.sub.3 N.sub.4 in the case that the Si.sub.3 N.sub.4 whiskers are mixed into Si.sub.3 N.sub.4 powder, and are used as a reinforcement of the Si.sub.3 N.sub.4 sintered body. Si.sub.3 N.sub.4 has two crystal phase, .alpha. type and .beta. type, which belong to the hexagonal system. The dimension of crystal lattices of both the crystal types is that a.sub.o =7.76 .ANG. and c.sub.o =5.62 .ANG. in the case of the .alpha. type, while a.sub.o =7.61 .ANG. and c.sub.o =2.91 .ANG. in the case of the .beta. type. The dimension of the crystal lattice in the c-axis direction is largely different between the .alpha. type and the .beta. type. .beta. is a high temperature phase which is changed from the .alpha. type at about 1,400.degree.-1,600.degree. C. The .alpha. type Si.sub.3 N.sub.4 powder having a high purity and a high sinterability is suitable for the raw material of the Si.sub.3 N.sub.4 sintered body. This material is considered to impart a high strength to the sintered body through phase transition from the .alpha. type to the .beta. type in the course of sintering. It is reported that when Si.sub.3 N.sub.4 whiskers abundant in the .alpha. type are added to such an .alpha. type Si.sub.3 N.sub.4 powder and the mixture is sintered, the whiskers themselves undergo the phase transition in the course of the sintering, so that the strain is caused between matrices to lower the strength of the sintered body. It is considered that the influence of the phase transition of the Si.sub.3 N.sub.4 whiskers similarly occurs in the case of the reinforcing of a sintered body other than Si.sub.3 N.sub.4. Thus, it has been long demanded to develop a method of manufacturing Si.sub.3 N.sub.4 whiskers composed mainly of the .beta. type.